The present invention is directed to a chair base adapted to support a column for supporting a chair seat and, in particular, to a chair base having a stepped hub to prevent creep of the column through the chair base.
Prior chair bases, of the type illustrated in Shepherd Products Design U.S. Pat. Des. Nos. 1312,011 and 321,299, have included a central hub from which a plurality of legs radially extend. The hubs in these prior chair bases generally include a bore extending vertically therethrough which forms a single conical wall portion within the hub. The conical wall portion of the hub is made entirely of plastic or includes a conical metal insert positioned within the bore of the hub. The bottom end of the column which is inserted into the bore of the chair base generally also includes a tapered conical wall adapted to engage the conical wall of the hub. As the tapered wall of the column is inserted into the bore of the hub the tapered wall of the column becomes wedged into engagement with the conical wall portion of the hub. This wedging effect of the column into the hub attaches the column to the chair base such that the top end of the column does not wiggle or deflect laterally.
Over time, as loads are repeatedly applied to the chair column, the bottom end of the column used with prior chair bases has been found to creep downwardly through the bore in the hub of the chair base from its originally installed position. As the column creeps downwardly, the conical wall of the column exerts increasingly greater forces on the hub and causes the formation of stress fractures in the hub wall thereby making the chair unsafe for further use as eventually a complete fracture of the hub will take place whereupon the column and attached chair may topple over. In those instances where the hub may not completely fracture, its internal diameter will continue to expand and the column will continue to creep downwardly until the bottom end of the column passes through the hub and hits the floor and thereafter drags therealong when the chair is moved horizontally. Even the use of steel sleeves as inserts within the bore of the hub has not been able to prevent the unwanted downward creep of the column through the hubs of prior chair bases.
The downward creep of the column through the hub of a chair base not only causes potential safety hazards due to the resulting stress fracturing of the hub and the potential sudden total fracturing of the hub, but also severely shortens the otherwise useful life of the chair base. There has therefore been a need for a chair base in which a column may be inserted and rigidly supported to prevent lateral deflection and which will also prevent the downward creep of the column through the hub of the chair base beyond a predetermined point.